1. Field of the Invention
This invention relates to an electrophotographic lithographic printing plate precursor made by an electrophotographic system and more particularly, it is concerned with an improvement in a photoconductive layer forming composition for the lithographic printing plate precursor.
2. Description of the Prior Art
A number of offset masters for directly producing printing plates have hitherto been proposed and some of them have already been put into practical use. Widely employed among them is a system in which a photoreceptor comprising a conductive support having provided thereon a photoconductive layer mainly comprising photoconductive particles, for example, of zinc oxide and a resin binder is subjected to an ordinary electrophotographic processing to form a highly lipophilic toner image on the surface of the photoreceptor, followed by treating the surface with an oil-desensitizing solution referred to as an etching solution to selectively render non-image areas hydrophilic and thus obtain an offset printing plate.
Requirements of offset masters for obtaining satisfactory prints include: (1) an original should be reproduced faithfully on the photoreceptor; (2) the surface of the photoreceptor has affinity with an oil-desensitizing solution so as to render non-image areas sufficiently hydrophilic, but, at the same time, has resistance to solubilization; and (3) a photoconductive layer having an image formed thereon is not released during printing and is well receptive to dampening water so that the non-image areas retain the hydrophilic properties sufficiently to be free from stains even upon printing a large number of prints.
It is known that these properties are affected by the ratio of zinc oxide to a resin binder in the photoconductive layer. For example, if the ratio of a binder resin to zinc oxide particles is decreased, oil-desensitivity of the surface of the photoconductive layer is increased to reduce background stains, but, on the other hand, the internal cohesion of the photoconductive layer per se is weakened, resulting in reduction of printing durability due to insufficient mechanical strength. If the ratio of a binder resin to zinc oxide particles is increased, on the other hand, printing durability is improved, but background staining becomes conspicuous. It is a matter of course that the background staining is a phenomenon associated with the degree of oil-desensitization achieved and it has been made apparent that the oil-desensitization of the photoconductive layer surface depends on not only the binder resin/zinc oxide ratio in the photoconductive layer, but also the kind of the binder resin used to a great extent.
For particular use as an offset master, occurrence of background stains due to insufficient oil-desensitivity presents a serious problem. In order to solve this problem, various resins for binding zinc oxide have been proposed to improve the oil-desensitization, including resins of weight average molecular weight (Mw) 1.8-10.times.10.sup.-4 and glass transition point (Tg) 10.degree.-80.degree. C. obtained by copolymerizing (meth)acrylate monomers and other monomers in the presence of fumaric acid in combination with copolymers of (meth)acrylate monomers and other monomers than fumaric acid, as disclosed in Japanese Patent Publication No. 31011/1975; terpolymers each containing a (meth)acrylic acid ester unit having a substituent having carboxylic acid group at least 7 atoms distant from the ester linkage, as disclosed in Japanese Patent Laid-Open Publication No. 54027/1978; tetra- or pentamers each containing an acrylic acid unit and hydroxyethyl (meth)acrylate unit, as disclosed in Japanese Patent Laid-Open Publication Nos. 20735/1979 and 202544/1982; terpolymers each containing a (meth)acrylic acid ester unit having an alkyl group having 6 to 12 carbon atoms as a substituent and a vinyl monomer containing carboxylic acid group, as disclosed in Japanese Patent Laid-Open Publication No. 046/1983; and the like. These resins function to improve the oil-desensitivity of photoconductive layers.
Nevertheless, evaluation of such resins as noted above for improving the oil-desensitization indicate that none of them is completely satisfactory in terms of stain resistance, printing durability and the like.
Furthermore, it has hitherto been studied to use resins having functional groups capable of forming hydrophilic groups through decomposition as such a binder resin, for example, those having functional groups capable of forming hydroxyl groups as disclosed in Japanese Patent Laid-Open Publication Nos. 195684/1987, 210475/1987 and 210476/1987 and those having functional groups capable of forming carboxyl groups as disclosed in Japanese Patent Laid-Open Publication No. 12669/1987.
These resins are those which form hydrophilic groups through hydrolysis or hydrogenolysis with an oil-desensitizing solution or dampening water used during printing. When using them as a binder resin for a lithographic printing plate precursor, it is possible to avoid various problems, e.g., deterioration of smoothness, deterioration of electrophotographic properties such as dark charge retention and photosensitivity, etc., which are considered to be caused by strong interaction of the hydrophilic groups and surfaces of photoconductive zinc oxide particles in the case of using resins intrinsically having hydrophilic groups per se, and at the same time, a number of prints with clear image quality and without background stains can be obtained, since the hydrophilic property of non-image areas rendered hydrophilic with an oil-desensitizing solution is further increased by the above described hydrophilic groups formed through decomposition in the resin to make clear the lipophilic property of image areas and the hydrophilic property of non-image areas and to prevent the non-image areas from adhesion of a printing ink during printing.
At the present time, in the electrophotographic lithographic printing, a higher efficiency has been required and in particular, it has been required to increase the speeds of plate making and etching and to obtain a print with a clear image quality, particularly free from background stains, from the start of printing, thus reducing loss of prints.
In the scanning exposing system using a semiconductor laser beam, furthermore, higher performances are required for static properties, in particular, dark charge retension and photosensitivity, since the exposing time is longer and the exposing intensity is more restricted than in the overall and simultaneously exposing system of the prior art using visible rays.
For such requirements is insufficient the above proposed offset printing plate using the binder resin capable of forming hydrophilic groups through decomposition with respect to the problems of increasing the etching speed and reducing the loss of prints.